The most common accepted definition is that an Assault rifle is a rifle that can be set to Fully automatic. This is NOT the Media's definition BTW.. A defense gun is a weapon used for defense, so really, almost any gun.. depending on the situation.. If you are carrying a Desert Eagle it may be considered MORE than a defense gun, but if you live on a patch of land along the southern boarder, it may be perfectly reasonable (notice I did not say reliable!)Hope it helps.